1. Field
Generally, this invention relates to mobile vehicles and more specifically to legged vehicles. This invention also may be used to improve the mobility of vehicles that use wheels, tracks and other driving appendages.
Classifications:                180/8.1 Motor Vehicles/Stepper        180/8.6 Motor Vehicles/Stepper with Alternatively Lifted Feet or Skid        
2. Prior Art
Many tasks require a vehicle or mobile robot capable of traversing a variety of terrains, including broken ground, gravel, and stairways. In many of these environments, a standard wheel is a hindrance. Early prior art in this field dealt primarily with agrarian needs, such as traction wheels for tractors, to provide greater traction than a standard wheel would allow (U.S. Pat. No. 1,144,373). In tasks such as stair climbing, a wheel's traction and surface engagement is crucial to prevent slippage. To address this issue, vehicles have been designed with variations on a standard wheel, such as one with lobed surfaces (U.S. Pat. No. 4,960,179) or one with spoke-like projections (U.S. Pat. No. 4,200,161), both allowing the vehicles to engage the surface to be climbed. A significant limitation of such designs, however, is that they are not easily adaptable for use on varying terrains, with some modification to the vehicle or its appendages typically required to even transition between climbing obstacles and moving on a level surface.
A further limitation of a standard wheel is its inability to roll over obstructions that are more than some portion of its radius. As larger and larger obstacles must be surmounted, the radius of the wheels required becomes absurd.
A patent for a toy vehicle with a non-standard wheel design (U.S. Pat. No. 3,529,479) exhibited two driving appendages consisting of a hub with spoke-like legs with an equiangular spacing. The vehicle rode on these legged-wheels and a tail-like protuberance that dragged on the ground. The vehicle was nominally able to traverse rugged surfaces. The device further contained a winch and a clutch mechanism for driving the winch, the legged-wheels, and for automatically switching between the winch and legged-wheels when a torque limit was reached. With no provision for dynamically or passively modifying the phase relationship of the legged-wheels, however, this device was not able to adjust its gait, limiting is climbing ability as well as its speed on level surfaces. Also, having only two driving appendages, its control and stability were limited. Since this device was designed for toy novelty, its climbing ability was of lesser importance.
As an alternative to wheeled vehicles, a variety of legged vehicles have been proposed (e.g., U.S. Pat. No. 5,121,805). Each leg on one of these vehicles is typically a complicated mechanism it its own right, with multiple degrees of freedom or joints. Most require several motors or other actuators to drive each leg. In place of, or in addition to, multiple actuators per leg, some designs require complicated linkages, clutches, and transmissions to drive the legs. Further, given the weight of most current designs, the motors must be outfitted with heavy gear trains in order to lift and propel the vehicle. In some cases (U.S. Pat. Nos. 4,502,556, 4,503,924), complicated mechanics are used to decouple the horizontal and vertical motion. While decoupling these motions reduces power consumption on level ground, this is only with the addition of mechanical complexity. The motors and mechanical devices found in these designs add significant weight to the vehicles, greatly limiting their speed and agility. Additionally, sophisticated computers, sensors, and control algorithms are required to make these vehicles function. These designs are thus hampered by their complexity, bulk, and weight, often resulting in unreliable, expensive, slow, and ponderous vehicles.
A recent example of prior art is US PA 2001/0054518, a 6 legged walking vehicle, using one motor per leg. The legs are actuated to produce an alternating tripod gate, wherein at least three legs are always in contact with the ground, leading to a stable pose. To produce this behavior, however, requires a sophisticated controlling computer to actuate each leg in the proper sequence, since they are not coupled mechanically. Each of these motors, then, must contain a position sensor (e.g., shaft encoder) to keep the legs in a coordinated gate. To reduce the number of actuators seen in other walking robot designs (i.e., 12-18) down to 6 also required that each motor follow a complicated trajectory, with high rotational accelerations and decelerations needed to swing each leg around to produce the tripod gate. Furthermore, while reducing the number of actuators down to 6 is a great improvement, 6 motors are still more than what is required to drive the vehicle in a tripod gate. The added weight of these motors reduces the payload of the vehicle and increases its cost.
While the prior art has advanced the field, current designs are hampered by their weight, cost, and complexity. A simple and robust design for a walking vehicle is thus needed and would further the relevant art.